Electrospun Ceramic-Polymer Composite As A Scaffold For Tissue Repair

ABSTRACT

The present invention relates to compositions and methods of preparing a three-dimensional matrix of micron sized electrospun fibers, wherein the electrospun fibers are formed from a electrospun composite comprising a bioactive ceramic component and a polymer component. The matrix provides an osteoconductive and osteoinductive scaffold supporting osteogenesis and thereby facilitates bone repair.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/944,587, entitled “Electrospun Ceramic-Polymer Composite As AScaffold for Tissue Repair,” filed Jun. 18, 2007, which is incorporatedherein by reference in its entirety.

GOVERNMENT SUPPORT

This work is supported at least in part by grants to Dr. Arinzeh. Thegovernment may have certain rights in this invention.

FIELD OF THE INVENTION

The present invention relates to compositions and methods of preparing athree-dimensional matrix of micron sized electrospun fibers, wherein theelectrospun fibers are formed from an electrospun composite comprising abioactive ceramic component and a polymer component. The matrix providesan osteoconductive and osteoinductive scaffold supporting osteogenesisand thereby facilitates bone repair.

BACKGROUND OF THE INVENTION

The repair of large bone defects resulting from trauma, metabolicdisorders, and tumor removal is a major medical challenge. Typically,such defects are treated with a bone allograft, where the terms“allograft” or “allogeneic transplant” are used interchangeably to referto situations in which transplanted cells, tissues, or organs aresourced from a genetically non-identical member of the same species.However, allografts lack osteoinductive factors necessary to acceleratenew bone growth and may carry the risk of disease transmission, sincesuch grafts typically are harvested from cadavers. Due to theselimitations, alternative strategies are needed.

Tissue engineering is one approach to the repair of large bone defectsthat has gained considerable interest. Tissue engineering is theapplication of principles and methods of engineering and life sciencestoward a fundamental understanding and development of biologicalsubstitutes to restore, maintain and improve human tissue functions.Bone regeneration may be achieved by the use of osteogenic cells and/orfactors to induce bone growth in combination with an appropriatescaffold to guide and support the laying down of new bone tissue.Optimally, a scaffold for bone tissue engineering should satisfy thefollowing minimum requirements: biocompatibility (meaning the ability tocoexist with living tissues or organisms without causing harm),osteoconductivity (meaning the ability to serve as a scaffold or matrixon which bone cells may attach, migrate and form new bone), porosity(meaning having minute openings, pores or holes that may be filled(permeated) by water, air or other materials), biodegradability (meaninghaving the ability to break down into harmless substances by the actionof living organisms) and mechanical integrity (meaning having theability to hold together and withstand chemical, physical, andbiological forces over time).

The term “bioceramic” refers to ceramic materials employed within thebody. Bioceramics employed within the body may be inert (meaning theyremain unchanged), resorbable (meaning they dissolve) or active (meaningthey may take part actively in physiological processes). Bioceramics maytake many forms, including, but not limited to, microspheres, thinlayers or coatings, porous networks, composites having a polymercomponent, and large well-polished surfaces. Direct use of ceramics forclinical applications has been limited because of their brittleness anddifficulty in shaping.

Generally, those of skill in the art combine one ceramic and one polymerto create scaffolds appropriate for bone tissue engineering. K. Rezwanet al., Biodegradable and bioactive porous polymer/inorganic compositescaffolds for bone tissue engineering, Biomaterials 27 (2006) 3413-3431.

Several methods have been used to synthesize tissue engineeringscaffolds based on polycaprolactone and polycaprolactone-tricalciumphosphate composites. For example, Zhou et al (In vitro bone engineeringbased on polycaprolactone and polycaprolactone-tricalcium phosphatecomposites, Polym Int 56 (2007) 333-342) used a fused depositionmodeling method to synthesize a composite of poly(ε-caprolactone) (PCL)and tricalcium phosphate (TCP). Although a foaming method has been usedto create hydroxyapatite-poly(L-lactic acid) ((HA)-(PLLA)) andβ-TCP-poly(lactic acid) (β-TCP-PLA) composites M. Montjovent et al.Biocompatibility of bioresorbable poly(L-lactic acid) compositescaffolds obtained by supercritical gas foaming with human fetal bonecells, Tissue Engineering 11 (2005) 1640-1649, the authors did notcreate a composite of PLA, HA, and β-TCP together. G. Georgiou et al.,Polylactic acid-phosphate glass composite foams as scaffolds for bonetissue engineering, J. Biomed. Mat. Res. Part B: Applied Biomaterials,published online Jul. 12, 2006, used a foaming or compression moldingmethod to synthesize a composite of PLA and a phosphate. See also, U.S.Pat. No. 5,626,861; U.S. Pat. No. 5,681,873, U.S. Pat. No. 5,766,618(the '618 patent), U.S. Pat. No. 5,955,529; U.S. Pat. No. 6,165,486;U.S. Pat. No. 6,306,424; U.S. Pat. No. 6,730,252; U.S. Pat. No.7,012,106; U.S. Pat. No. 7,022,522. None of these contemplate the use ofelectrospinning as a method to synthesize a scaffold containing ceramicpolymer composites.

Electrospinning, another method that has been used to synthesizepolymeric tissue engineering scaffolds, applies a high voltage to anejectable polymer solution. The basic principle behind this process isthat an electric voltage sufficient enough to overcome the surfacetension of a polymeric solution causes the polymer droplets to elongateso that the polymer is splayed randomly as very fine fibers, which whencollected on a grounded metal plate, form non-woven mats. Traditionally,electrospinning has yielded nonwoven mats (also called matrices andscaffolds) of nanometer sized fiber diameters and nanometer sized porediameters. However, in order for cells to infiltrate into a scaffold andproliferate, micron sized fiber diameters and micron sized porediameters are optimal. Since the diameter of a cell is approximately 10μm to 20 μm, pore sizes at the cellular level or above are needed toallow for cell infiltration.

Polymer and calcium phosphate ceramic composites used in conventionalscaffold-forming techniques are not easily adaptable to theelectrospinning method. The parameters of voltage, flow rate, needlegauge size, distance to collection plate, and polymer solutionconcentration during processing need to be optimized to achieve fibrousmats. When combining a polymer with a ceramic in solution, in additionto optimizing these parameters, the homogeneity of the polymer-ceramicmixtures must be ensured. Moreover, the literature in this field doesnot provide sufficient guidance to enable one of skill in the art oftissue engineering to adapt polymer and ceramic composites to theelectrospinning method using routine experimentation.

Previous work to develop scaffold materials for tissue engineering byelectrospinning using polycaprolactone (PCL) or hydroxyapatite (HA) hasproduced mats containing nanometer sized fiber diameters havingnanosized pore diameters in the mat. Such mats are not optimal forosteogenesis, because these pore diameters are below the preferred rangeof pore sizes for cell infiltration. See e.g. H. Yoshimoto et al., Abiodegradable nanofiber scaffold by electrospinning and its potentialfor bone tissue engineering, Biomaterials 24 (2003) 2077-2082; M. Shinet al., In vivo bone tissue engineering using mesenchymal stem cells ona novel electrospun nanofibrous scaffold, Tissue Engineering 10 (2004)33-41; C. Li et al., Electrospun silk-BMP-2 scaffolds for bone tissueengineering, Biomaterials 27 (2006) 3115-3124. HA also has beenelectrospun with PCL alone (P. Wutticharoenmongkol et al., Preparationand characterization of novel bone scaffolds based on electrospunpolycaprolactone fibers filled with nanoparticles, Macromol. Biosci. 6(2006) 70-77) and with PCL and collagen (J. Venugopal et al.,Biocomposite nanofibres and osteoblasts for bone tissue engineering,Nanotechnology 18 (2007) 1-8).

Likewise, a number of patents have disclosed an electrospinning methodfor production of polymer nanofibers. These nanofiber mats havenanosized pore diameters in the mat, which are below the desired rangeof pore sizes necessary for cell infiltration. See, for example, U.S.Pat. No. 6,689,166; U.S. Pat. No. 6,790,528; U.S. Published Pat. App.No. 2004/0018226; U.S. Published Pat. App. No. 2006/0204539; U.S.Published Pat. App. No. 2006/0128012.

In order for a biodegradable scaffold to be successful, the materialmust have a rate of degradation that is commensurate with the growth ofnew bone. Ideally, the scaffold should degrade slowly enough to maintainstructural support during the initial stages of bone formation, but fastenough to allow space for continuous growth of new bone. Previousstudies have demonstrated the potential of biphasic compositions of HAand β-TCP ceramics for bone tissue engineering applications. One majoradvantage is that their rate of degradation correlates with bone tissueformation.

The present invention, which addresses this problem, providescompositions and methods of preparing a three-dimensional matrix ofmicron sized electrospun fibers, wherein the electrospun fibers areformed from a electrospun composite comprising a bioactive ceramiccomponent and a polymer component. The matrix provides anosteoconductive and osteoinductive scaffold supporting osteogenesis andthereby may facilitate bone repair.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides an implantable scaffoldfor use in tissue engineering comprising a three-dimensional matrix ofmicron sized electrospun fibers, wherein the electrospun fibers areformed from an electrospun composite comprising a bioactive ceramiccomponent and a polymer component. According to one embodiment, theelectrospun composite of the implantable scaffold contains at least 10%by weight of the ceramic component. According to another embodiment, theelectrospun composite of the implantable scaffold contains at leastabout 60% by weight of the polymer component. According to anotherembodiment, the electrospun composite of the implantable scaffoldcontains at least 10% by weight of the ceramic component and at leastabout 60% by weight of the polymer component. According to anotherembodiment, the bioactive ceramic component of the electrospun compositeof the implantable scaffold contains two calcium phosphate ceramicsubstances for every polymer in the polymer component of the composite.According to another embodiment, the calcium phosphate ceramicsubstances of the bioactive ceramic component of the electrospuncomposite are selected from the group consisting of tetracalciumphosphate, amorphous calcium phosphate, alpha-tricalcium phosphatebeta-tricalcium phosphate and hydroxyapatite. According to anotherembodiment, the two calcium phosphate ceramic substances of thebioactive ceramic component of the electrospun composite arehydroxyapatite and tricalcium phosphate. According to anotherembodiment, the bioactive ceramic component of the electrospun compositecomprises 20% hydroxyapatite and 80% tricalcium phosphate. According toanother embodiment, the polymer component of the electrospun compositeis at least one polymer selected from the group consisting of anondegradable polymer and a a degradable polymer. According to anotherembodiment, the nondegradable polymer is selected from the groupconsisting of a polyurethane, a polyvinylidine fluoride, and apolyvinylidine fluoride trifluoroethylene. According to anotherembodiment, the degradable polymer is selected from the group consistingof poly(lactic acid-glycolic acid), poly(lactic acid), poly(glycolicacid), a poly(orthoester), a poly(phosphazene), a polycaprolactone, apolyamide, a polysaccharide, and a collagen. According to anotherembodiment, the polymer of the polymer component of the electrospuncomposite is polycaprolactone. According to another embodiment, thethree dimensional matrix of electrospun fibers of the implantablescaffold comprises micron-sized pores.

In another aspect, the present invention provides a composition forpreparing an implantable osteogenic three-dimensional matrix of micronsized electrospun fibers, the composition comprising an electrospuncomposite containing a bioactive ceramic component and a polymercomponent. According to one embodiment, the electrospun composite of thecomposition contains at least 10% by weight of the ceramic component.According to another embodiment, the electrospun composite of thecomposition contains at least about 60% by weight of the polymercomponent. According to another embodiment, the electrospun composite ofthe composition contains at least 10% by weight of the ceramic componentand at least about 60% by weight of the polymer component. According toanother embodiment, the bioactive ceramic component of the electrospuncomposite of the composition contains two calcium phosphate ceramicsubstances for every polymer in the polymer component of the composite.According to another embodiment, the calcium phosphate ceramicsubstances of the bioactive ceramic component of the electrospuncomposite of the composition are selected from the group consisting oftetracalcium phosphate, amorphous calcium phosphate, alpha-tricalciumphosphate, beta-tricalcium phosphate, and hydroxyapatite. According toanother embodiment, the two calcium phosphate ceramic substances of thebioactive ceramic component of the electrospun composite of thecomposition are hydroxyapatite and tricalcium phosphate. According toanother embodiment, the bioactive ceramic component of the electrospuncomposite of the composition comprises 20% hydroxyapatite and 80%tricalcium phosphate. According to another embodiment, the polymercomponent of the electrospun composite is at least one polymer selectedfrom the group consisting of a nondegradable polymer and a a degradablepolymer. According to another embodiment, the nondegradable polymer isselected from the group consisting of a polyurethane, a polyvinylidinefluoride, and a polyvinylidine fluoride trifluoroethylene. According toanother embodiment, the degradable polymer is selected from the groupconsisting of poly(lactic acid-glycolic acid), poly(lactic acid),poly(glycolic acid), a poly(orthoester), a poly(phosphazene), apolycaprolactone, a polyamide, a polysaccharide, and a collagen.According to another embodiment, the polymer of the polymer component ofthe electrospun composite of the composition is polycaprolactone.

In another aspect, the present invention provides a method of preparinga osteoinductive scaffold to facilitate bone repair, the methodcomprising the steps: (a) preparing a ceramic-polymer compositecomprising a bioactive ceramic component and a polymer component; (b)electrospinning the ceramic-polymer composite, and thereby (c)depositing a three-dimensional nonwoven matrix of electrospun fiberscomprising the ceramic-polymer composite on a collector. According toone embodiment of the method, the electrospun composite of step (a)contains at least 10% by weight of the ceramic component. According toanother embodiment of the method, the electrospun composite of step (a)contains at least about 60% by weight of the polymer component.According to another embodiment of the method, the electrospun compositecontains at least 10% by weight of the ceramic component and at leastabout 60% by weight of the polymer component. According to anotherembodiment of the method, the bioactive ceramic component of thecomposite of step (a) contains two calcium phosphate ceramic substancesfor every polymer in the polymer component of the composite. Accordingto another embodiment of the method, the calcium phosphate ceramicsubstances of the bioactive ceramic component of the composition of step(a) are selected from the group consisting of tetracalcium phosphate,amorphous calcium phosphate, alpha-tricalcium phosphate, beta-tricalciumphosphate, and hydroxyapatite. According to another embodiment of themethod, the two calcium phosphate ceramic substances of the bioactiveceramic component of step (a) are hydroxyapatite and tricalciumphosphate. According to another embodiment of the method, the bioactiveceramic component of the composite of step (a) comprises 20%hydroxyapatite and 80% tricalcium phosphate. According to anotherembodiment of the method, the polymer component of the composite of step(a) is at least one polymer selected from the group consisting of anondegradable polymer and a degradable polymer. According to anotherembodiment, the nondegradable polymer is selected from the groupconsisting of a polyurethane, a polyvinylidine fluoride, and apolyvinylidine fluoride trifluoroethylene. According to anotherembodiment, the degradable polymer is selected from the group consistingof poly(lactic acid-glycolic acid), poly(lactic acid), poly(glycolicacid), a poly(orthoester), a poly(phosphazene), a polycaprolactone, apolyamide, a polysaccharide, and a collagen. According to anotherembodiment of the method, the polymer of the polymer component of thecomposite of step (a) is polycaprolactone. According to anotherembodiment of the method, the electrospun fibers of step (c) are micronsized. According to another embodiment of the method, the matrix ofelectrospun fibers in step (c) comprises micron-sized pores. Accordingto another embodiment of the method, the method further comprises thesteps: (d) seeding the three-dimensional nonwoven matrix of electrospunfibers with isolated differentiable human mesenchymal cells orosteoblasts; and (e) growing the differentiable human mesenchymal cellsor osteoblasts on the three-dimensional nonwoven matrix of electrospunfibers so that the differentiable human mesenchymal cells or osteoblastsdifferentiate into a mature cell phenotype on the scaffold.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagrammatic representation of the electrospinning equipmentused herein.

FIG. 2A shows a scanning electron micrograph of one embodiment of anelectrospun polycaprolactone containing 10% nanohydroxyapatite. FIG. 2Bshows corresponding energy dispersive x-ray mapping of calcium levels ofthe mat shown in FIG. 2A as an indicator of the location ofhydroxyapatite.

FIG. 3A shows a scanning electron micrograph of one embodiment of anelectrospun mat containing 30% HA-TCP formed using methylene chloride assolvent. FIG. 3B shows corresponding energy dispersive x-ray mapping ofcalcium levels of the mat shown in FIG. 3A as an indicator of thelocation of hydroxyapatite.

FIG. 4A shows a scanning electron micrograph of one embodiment of anelectrospun mat containing 30% HA-TCP formed using methylene chlorideand dimethylformamide as solvent. FIG. 4B shows corresponding energydispersive x-ray mapping of calcium levels of the mat shown in FIG. 4Aas an indicator of the location of hydroxyapatite.

FIG. 5 shows a thermogravimetric analysis of electrospun samples madeusing dimethylformamide and methylene chloride as solvent (ProcedureII).

FIG. 6 shows a thermogravimetric degradation profile of electrospunsamples formed using dimethylformamide and methylene chloride as solvent(Procedure II).

FIG. 7 shows the crystallization behavior of electrospun compositesformed using methylene chloride (MC) as solvent. The heat of fusion(H_(f)) for crystallization and crystallization temperature values arelabeled on the graphs.

FIG. 8 shows the crystallization behavior of electrospun compositesformed using methylene chloride (MC) and dimethylformamide (DMF) assolvent (Procedure II). The heat of fusion (H_(f)) for crystallizationand crystallization temperature values are labeled on the graphs.

FIG. 9 shows the melting behavior of electrospun composites formed usingmethylene chloride as solvent. (Procedure I). The heat of fusion (H_(f))for crystallization and crystallization temperature values are labeledon the graphs.

FIG. 10 shows the melting behavior of electrospun composites formedusing methylene chloride (MC) and dimethylformamide (DMF) as solvent(Procedure II). The heat of fusion (H_(f)) for crystallization andcrystallization temperature values are labeled on the graphs.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an electrospun scaffold of bioactiveceramic materials having micron sized fiber diameters and pores and amethod of preparing such scaffolds. The pore size diameters of themicron sized fibers in the scaffold provide for improved cellinfiltration, aggregation, and tissue formation throughout the scaffoldwhen compared with nanosized fibers.

As used herein, the term “Beta-Tricalcium Phosphate” (“β-TCP”) refers toa synthetic material whose formula is Ca₃(PO₄)₂.

As used herein, the terms “bioactive” and “bioactivity” are usedinterchangeably to refer to any effect on, interaction with, or responsefrom living tissue.

As used herein, the term “biocompatible material” refers to a materialthat the body generally accepts without a major immune response, whichis capable of implantation in biological systems, for example, tissueimplantation, without causing excessive fibrosis or rejection reactions.As used herein, the term “biodegradable” refers to the ability of asubstance or material to break down into harmless substances by theaction of living organisms.

The term “collagen” refers to any of a family of extracellular, closelyrelated proteins occurring as a major component of connective tissue,giving it strength and flexibility. At least 14 types exist, eachcomposed of tropocollagen units that share a common triple-helical shapebut that vary somewhat in composition between types, with the typesbeing localized to different tissues, stages, or functions. In sometypes, including the most common, Type I, the tropocollagen rodsassociate to form fibrils or fibers; in other types the rods are notfibrillar but are associated with fibrillar collagens, while in othersthey form nonfibrillar, nonperiodic but structured networks.Tropocollagen, the basic structural unit of collagen comprises a helicalstructure consisting of three polypeptide chains, each chain composed ofabout a thousand amino acids, coiled around each other to form a spiraland stabilized by inter- and intrachain covalent bonds. It is rich inglycine, which occurs as nearly one residue out of three, as well asproline, hydroxyproline, and hydroxylysine; the last two rarely occur inother proteins.

The terms “microscale fiber” or “micron sized fiber” are usedinterchangeably to refer to fibers whose diameter ranges from about 1micrometer (10⁻⁶ m) to about 1000 micrometers. The terms “nanoscalefiber” or “nano sized fiber” are used interchangeably to refer to fiberswhose diameter ranges from about 1 nanometer (10⁻⁹ m) to about 1000nanometers.

The term “osteoconduction” or “osteoconductive” as used herein refers toa material having the ability to serve as a scaffold on which bone cellsmay attach, migrate and form new bone.

The term osteoinduction or “osteoinductive” as used herein refers to amaterial having the ability to induce bone to grow.

The term “polymer” as used herein refers to a macromolecule formed bythe chemical union of five or more identical combining units calledmonomers. In most cases, the number of monomer is quite large and oftenis not precisely known. In synthetic polymers, this number may becontrolled to a predetermined extent. Combinations two, three, or fourmonomers are called, respectively, dimers, trimers, and tetramers, andare known collectively as oligomers. Polymers may be inorganic (e.g.,siloxane, sulfur chains, black phosphorus, boron-nitrogen, silicones) ororganic (meaning containing carbon). Organic polymers may be natural[e.g., polysaccharides, such as starch, cellulose, pectin, seaweed gums,vegetable gums; polypeptides, such as casein, albumin, globulin,keratin, insulin, DNA; and hydrocarbons], synthetic [such asthermoplastics (unvulcanized elastomers, nylon, polyvinyl chloride,linear polyethylene, polystyrene, polypropylene, polyurethane, acrylateresins); thermosetting (e.g., vulcanized elastomers, crosslinkedpolyethylene, phenolics, alkyds, polyesters), and semisynthetic (e.g.,cellulosics, such as rayon, methylcellulose, cellulose acetate; andmodified starches)].

The term “polyamide” refers to a polymer containing monomers joined byamide linkages (—CONH—) or peptide bonds. A peptide bond is a chemicalbond formed between two molecules when the carboxyl of one moleculereacts with the amino of the other molecule, releasing a molecule ofwater. The resulting CO—NH bond is called a peptide bond, and theresulting molecule is called an amide. Polyamides may occur bothnaturally (e.g., proteins such as wool and silk) and may be madeartificially (e.g., nylons, aramids, and sodium poly(aspartate).

The term “homopolymer” refers to a natural or synthetic polymer derivedfrom a single monomer.

The terms “polycaprolactone”, 6-Caprolactone polymer, and “PCL” are usedinterchangeably to refer to a biodegradable polyester having themolecular formula (C₆H₁₀O₂)_(n), and having a molecular weight of about80,000 daltons. PCL may be obtained commercially, for example, fromSigma-Aldrich.

The terms “poly(glycolic acid)”, polyglycolide, and “PGA” are usedinterchangeably herein to refer to a biodegradable, thermoplasticpolymer and the simplest linear, aliphatic polyester. PGA may beobtained commercially, for example, from Sigma-Aldrich.

A “polylactide” is a biodegradable polymer derived from lactic acid.Poly(lactide) or PLA exists in two stereo forms, signified by a D or Lfor dexorotary or levorotary, or by DL for the racemic mix. The term“PLLA” refers to the biodegradable aliphatic polyester homopolymer polyL-lactic acid. PLLA may be obtained commercially, for example, fromAlkermes, Inc.

The terms poly (lactic acid-glycolic acid), poly(D,L-lactide-c-glycoside), and PLGA are used interchangeably to refer toa copolymer of polylactic acid and glycolic acid. PLGA may be obtainedcommercially, for example, from Alkermes, Inc.

The term “poly(orthoester)” refers to a synthetic material having themolecular structure

which is hydrophobic with hydrolytic linkages that are acid-sensitive,but stable to base. See J. C. Middleton, A. J. Tipton, Biomaterials 21:2335-46 (2000).

A phosphazene is a ring or chain polymer that contains alternatingphosphorus and nitrogen atoms with two substituents on each phosphorusatom such that a phosphorus atom is covalently linked to a nitrogen atomby a double bond and to three other atoms or radicals by single bonds.Two examples are hexachlorocyclotriphosphazene, a cyclic compound, whosechemical formula is more descriptively written as (PNCl₂)₃ andbis(triphenylphosphine)iminium chloride (PPNCl). The term“poly(phosphazene)” refers to polymers having a phosphazene repeatingunit and the general structure (—RR′P═N—).

A “polysaccharide” is a long-chain natural or synthetic polymer made upof linked simple sugars (monosaccharides) such as glucose and closelyrelated molecules. Two monosaccharide molecules may be joined by aglycosidic bond to form a disaccharide, as, for instance, in the linkageof glucose and fructose to create sucrose. More complicatedpolysaccharides such as starch, glycogen, cellulose or chitin consist ofnumerous monosaccharide units joined by glycosidic bonds.

The term “porous” as used herein relates to having minute openings,pores, or holes that may be filled (permeated) by water, air or othermaterials.

As used herein, the term “stem cells” refers to undifferentiated cellshaving high proliferative potential with the ability to self-renew thatmay migrate to areas of injury and may generate daughter cells that mayundergo terminal differentiation into more than one distinct cellphenotype. These cells have the ability to differentiate into variouscells types and thus promote the regeneration or repair of a diseased ordamaged tissue of interest. The term “cellular differentiation” refersto the process by which cells acquire a cell type. The term “progenitorcell” as used herein refers to an immature cell in the bone marrow thatmay be isolated by growing suspensions of marrow cells in culture disheswith added growth factors. Progenitor cells are referred to ascolony-forming units (CFU) or colony-forming cells (CFC). The specificlineage of a progenitor cell is indicated by a suffix, such as, but notlimited to, CFU-F (fibroblastic).

As used herein, the terms “osteoprogenitor cells,” “mesenchymal cells,”“mesenchymal stem cells (MSC),” or “marrow stromal cells” are usedinterchangeably to refer to multipotent stem cells that differentiatefrom CFU-F cells capable of differentiating along several lineagepathways into osteoblasts, chondrocytes, myocytes and adipocytes. Whenreferring to bone or cartilage, MSCs commonly are known asosteochondrogenic, osteogenic, chondrogenic, or osteoprogenitor cells,since a single MSC has shown the ability to differentiate intochondrocytes or osteoblasts, depending on the medium.

The term “chondrocytes” as used herein refers to cells found incartilage that produce and maintain the cartilaginous matrix. From leastto terminally differentiated, the chondrocytic lineage is (i)Colony-forming unit-fibroblast (CFU-F); (ii) mesenchymal stemcell/marrow stromal cell (MSC); (3) chondrocyte. The term“chondrogenesis” refers to the formation of new cartilage from cartilageforming or chondrocompetent cells.

The term “osteoblasts” as used herein refers to cells that arise whenosteoprogenitor cells or mesenchymal cells, which are located near allbony surfaces and within the bone marrow, differentiate under theinfluence of growth factors. Osteoblasts, which are responsible for bonematrix synthesis, secrete a collagen rich ground substance essential forlater mineralization of hydroxyapatite and other crystals. The collagenstrands to form osteoids: spiral fibers of bone matrix. Osteoblastscause calcium salts and phosphorus to precipitate from the blood, whichbond with the newly formed osteoid to mineralize the bone tissue. Onceosteoblasts become trapped in the matrix they secrete, they becomeosteocytes. From least to terminally differentiated, the osteocytelineage is (i) Colony-forming unit-fibroblast (CFU-F); (ii) mesenchymalstem cell/marrow stromal cell (MSC); (3) osteoblast; (4) osteocyte. Theterm “osteogenesis” refers to the formation of new bone from boneforming or osteocompetent cells.

Although the lineage of adipocytes is still unclear, it appears thatMSCs may differentiate into two types of lipoblasts, one that give riseto white adipocytes and the other to brown adipocytes. Both types ofadipocytes store fat.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. Although any methods andmaterials similar or equivalent to those described herein also may beused in the practice or testing of the present invention, the preferredmethods and materials are now described. All publications mentionedherein are incorporated herein by reference to disclose and describe themethods and/or materials in connection with which the publications arecited.

It must be noted that as used herein and in the appended claims, thesingular forms “a”, “and”, and “the” include plural references unlessthe context clearly dictates otherwise. All technical and scientificterms used herein have the same meaning. Efforts have been made toensure accuracy with respect to numbers used (e.g. amounts, temperature,etc.) but some experimental errors and deviations should be accountedfor. Unless indicated otherwise, parts are parts by weight, molecularweight is weight average molecular weight, temperature is in degreesCentigrade, and pressure is at or near atmospheric.

Where a range of values is provided, it is understood that eachintervening value, to the tenth of the unit of the lower limit unlessthe context clearly dictates otherwise, between the upper and lowerlimit of that range and any other stated or intervening value in thatstated range is encompassed within the invention. The upper and lowerlimits of these smaller ranges which may independently be included inthe smaller ranges is also encompassed within the invention, subject toany specifically excluded limit in the stated range. Where the statedrange includes one or both of the limits, ranges excluding either bothof those included limits are also included in the invention.

The publications discussed herein are provided solely for theirdisclosure prior to the filing date of the present application. Nothingherein is to be construed as an admission that the present invention isnot entitled to antedate such publication by virtue of prior invention.Further, the dates of publication provided may be different from theactual publication dates which may need to be independently confirmed.

EXAMPLES

The following examples are put forth so as to provide those of ordinaryskill in the art with a complete disclosure and description of how tomake and use the present invention, and are not intended to limit thescope of what the inventors regard as their invention nor are theyintended to represent that the experiments below are all or the onlyexperiments performed.

Example 1 The Fabrication of Tissue Engineering Scaffolds

The Electrospinning Process

Electrospinning is a fiber forming technique that relies on chargeseparation to produce nano- to microscale fibers, which typically form anon-woven matrix. The terms “nonwoven matrix”, “nonwoven mesh” or“nonwoven scaffold” are used interchangeably herein to refer to amaterial comprising a randomly interlaced fibrous web of fibers.Generally, individual electrospun fibers have large surface-to-volumeand high aspect ratios resulting from the smallness of their diameters.These beneficial properties of the individual fibers are furtherenhanced by the porous structure of the non-woven fabric, which allowsfor cell infiltration, cell aggregation, and tissue formation.

The electrospinning process is affected by varying the electricpotential, flow rate, solution concentration, capillary-collectordistance, diameter of the needle, and ambient parameters liketemperature. Therefore, it is possible to manipulate the porosity,surface area, fineness and uniformity, diameter of fibers, and thepattern thickness of the matrix.

While the following is a description of a preferred embodiment, thepresent invention includes other protocols that achieve the sameresults.

One nonlimiting example of an apparatus and method used forelectrospinning has been disclosed in U.S. application Ser. No.11/291,701, which is incorporated herein by reference in its entirety.FIG. 1 is a diagrammatic representation of the electrospinning setupused herein, which is comprised of a syringe pump containing a 20 gaugeneedle. The syringe pump was mounted on a robotic arm in order tocontrol the splaying of fibers on the collector. An electricallygrounded stainless steel plate of dimensions 15 cm×30 cm was used as thecollector. The syringe pump was filled with the polymer-ceramicsolution, and a constant flow rate of 0.10 ml/min was maintained usingthe syringe pump. The positive output lead of a high voltage powersupply (Gamma High Voltage, Inc.) was attached to the needle, and a 17kvolt voltage was applied to the solution. The collector-to-needledistance was 15 cm. When the charge of the polymer at increasing voltageexceeded the surface tension at the tip of the needle, the polymersplayed randomly as fibers. These were collected as nonwoven mats on thegrounded plate.

This electrospinning technique is used to prepare a fibrous matrixcomprising a ceramic-polymer composite according to the presentinvention. The term “composite” as used herein refers to a complexmaterial, in which two or more distinct substances combine to producestructural or functional properties not present in any individualcomponent. The composite of the present invention is created bycombining a bioactive ceramic component with a polymer component.

The polymer component may be any natural or synthetic biocompatiblepolymer that may be dissolved in an organic (meaning carbon-containing)solvent for electrospinning according to the present invention. Whileany solvent generally useful to prepare a polymer solution may be usedfor this purpose, the fiber size, pore size, and polymer structure ofthe ceramic-polymer mats formed according to the present invention aredetermined by the solvent used to form the ceramic-polymer mats.Examples of suitable solvents include, but are not limited to,dimethylsulfoxide (DMSO), ethyl acetate, benzene, 2-butanone, carbontetrachloride, n-heptane, n-hexane, n-pentane, methylene chloride,dimethylformamide, chloroform, formic acid, ethyl formate, acetic acid,hexafluoroisopropanol, cyclic ethers, acetone, C₂-C₅ alcohol acetates,1-4 dioxane, and combinations thereof. In some embodiments, the solventis selected to be noncytotoxic.

A solution generally is considered as a homogeneous mixture of two ormore substances; it is frequently, though not necessarily, a liquid. Ina solution, the molecules of the solute (or dissolved substance) areuniformly distributed among those of the solvent. A suspension is adispersion (mixture) in which a finely-divided species is combined withanother species, with the former being so finely divided and mixed thatit doesn't rapidly settle out. In everyday life, the most commonsuspensions are those of solids in liquid water.

Examples of polymers that may be incorporated into the composite of thepresent invention include, without limitation, nondegradable polymers(such as polyurethanes (meaning a thermoplastic polymer produced by thereaction of polyisocyanates with linear polyesters or polyetherscontaining hydroxyl groups), polyvinylidine fluoride ((—H₂C═CF₂—)_(n)),and polyvinylidine fluoride trifluoroethylene) and degradable polymers(such as poly(lactic acid-glycolic acid), poly(lactic acid),poly(glycolic acid), a poly(orthoester), a poly(phosphazene),poly(caprolactone), a polyacrylamide, and collagen.

As used herein, the term “bioactive ceramic” includes, but is notlimited to, such substances as hydroxyapatite, tricalcium phosphate andbioactive glasses, each of which has superior osteoconductiveproperties. There are several calcium phosphate ceramics that areconsidered biocompatible. Some of these materials include, in order ofsolubility, tetracalcium phosphate (Ca₄P₂O₉)>amorphous calciumphosphate>alpha-tricalcium phosphate (Ca₃(PO₄)₂>beta-tricalciumphosphate (“β-TCP”) (Ca₃PO₄)₂>>hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂).

The terms “hydroxylapatite”, “hydroxyapatite” and “HA” are usedinterchangeably to refer to a mineral that is the major constituent ofbone and tooth mineral. It is a finely divided, crystalline,nonstoichiometric material rich in surface ions (including carbonate,magnesium, and citrate ions). It is thermodynamically stable atphysiological pH (meaning it does not break down under physiologicalconditions) and may form strong chemical bonds with surrounding bone.

In some embodiments, electrospun composites according to the presentinvention contain about 10% to about 40% by weight of a ceramiccomponent and about 60% to about 90% by weight of a polymer. In someembodiments, the electrospun composites contain at least 10%, at least15%, at least 20%, at least 25%, at least 30%, at least 35%, or at least40% by weight of a ceramic component. In some embodiments, theelectrospun composites contain no more than 40%, no more than 35%, nomore than 30%, no more than 25%, no more than 20%, no more than 15%, orno more than 10% by weight of a ceramic component. In some embodiments,the electrospun composites contain at least 60%, at least 65%, at least70%, at least 75%, at least 80%, at least 85%, or at least 90% of apolymer. In some embodiments, the electrospun composites contain no morethan 90%, no more than 85%, no more than 80%, no more than 75%, no morethan 70%, no more than 65%, or no more than 60% by weight of a polymer.

According to some embodiments of the present invention, the bioactiveceramic component of the composite comprises both hydroxyapatite andtricalcium phosphate. See, Wutticharoenmongkol P et al. J NanosciNanotechnol. (2006) 6(2): 514-522; Thomas V et al. J NanosciNanotechnol. (2006). 6(2): 487-493. According to some such embodiments,the composite comprises two calcium phosphate ceramics for each polymer.According to one embodiment of the present invention, a composition ofbioactive ceramics comprising 20% hydroxyapatite:80% beta-tricalciumphosphate (“20/80 HA/β-TCP”) is combined with a polymer, such aspolycaprolactone (PCL), and electrospun. Bioactive ceramic compositionscomprising 20/80 HA/β-TCP have been shown to promote stem celldifferentiation into osteoblasts (Livingston Arinzeh, T. L., Tran, T.,McAlary, J., Dalcusi, G. 2005. Comparative osteogenic activity ofvarious calcium phosphate ceramic compositions in vitro and in vivo.Biomaterials. 26(17), pp. 3631-3638)). 20/80 HA/β-TCP also strikes anoptimal balance between supporting bone formation while degradingexisting bone to allow for bone replacement.

The solution of polymer is prepared by weighing the required amount ofpolymer and dissolving it in a volume of solvent. In one embodiment,methylene chloride is used as the solvent. In another embodiment,methylene chloride and dimethylformamide in the ratio of 80:20 was usedas the solvent.

The concentration of the final solution depends on the concentrationthat is required. In some embodiments, for example, methylene chlorideis used as the solvent. In one such embodiment, 10 ml of methylenechloride (density is 1.3255 g/cm³) is used to dissolve 1 g of polymer.In another embodiment, 10 ml of methylene chloride is used to dissolve 2g of polymer. In another embodiment, 10 ml of methylene chloride is usedto dissolve 0.5 g of polymer. In another embodiment, 10 ml of methylenechloride is used to dissolve 1.5 g of polymer.

The solution is stirred using a magnetic stirrer overnight at roomtemperature. To make a composite material of polymer and ceramic, therequired amount of ceramic is weighed and added to the polymer solutionin a beaker. Hydroxyapatite having an average particle diameter of about100 nm and β-TCP having an average particle diameter of about 100 nm,may be purchased from Berkeley Advanced Biomaterials. The mixture isstirred manually using a glass stirrer for a few minutes and then pouredinto the syringe for electrospinning. The organic solvent substantiallyevaporates during the electrospinning process, resulting in less thanabout 1% of the solvent remaining in the final product.

Composites having ceramic loadings from about 10% to about 50% by weightwere fabricated. For example, a 20/80 HA/TCP polymer composite isprepared by preparing the required amount of the ceramic component andthen adding it to the polymer solution. The ceramic component isprepared by mixing 20 weight percent HA and 80 weight percent of βTCP.The required amount of the ceramic component is added to 10 ml of thepolymer solution and stirred for a few minutes prior to loading thecomposite into the syringe for electrospinning.

The bioactive ceramic-polymer composite of the present inventionproduces a fibrous mat that has micron sized fiber diameters and micronsized pore diameters. This increase in fiber and pore diameter size fromthe nano sized fibers to the micron sized fibers improves cellinfiltration, aggregation, and tissue formation throughout the scaffold,making it optimal for tissue engineering. In some such embodiments, themicroscale fibers have a diameter ranging from about 1 micrometer toabout 50 micrometers. In some embodiments, the diameter of themicrofibers averaged from about 1 micrometer to about 5 micrometers.

Morphology and Elemental Analysis

The morphology of the electrospun mats was studied using LEO 1530scanning electron microscopy (SEM). The estimate of fiber size ofelectrospun scaffolds was determined from the SEM images using ImageJsoftware. SEM-EDXA was performed on the sample to confirm the presenceof calcium and phosphorous in the electrospun mats. Mapping for calciumwas performed on the electrospun sample to determine the distribution ofthe ceramic in the polymer mat. An accelerating voltage of 10 KV voltageand a working distance of 10 mm were used for the SEM EDXA and mappinganalysis.

Non-woven mats may be electrospun with ceramic loading up to 50% byweight using methylene chloride as solvent according to the presentinvention.

FIG. 2A shows a scanning electron micrograph of one embodiment of anelectrospun polycaprolactone containing 10% nanohydroxyapatite formedusing methylene chloride as solvent. Fiber diameters were about 1 micronto about 5 microns and pore sizes were at least about 20 microns indiameter.

Energy Dispersive X-ray Spectroscopy (EDS) is an analytical techniquethat utilizes an electron beam to impact on the sample. This producesx-rays that are characteristic of the elements found on the sample. EDSmay be coupled with several applications, including Scanning ElectronMicroscopy (SEM), Transmission Electron Microscopy (TEM) and ScanningTransmission Electron Microscopy (STEM).

FIG. 2 b shows an energy-dispersive x-ray map of the fibers shown inFIG. 2 a. The location of the hydroxyapatite may be identified by thecalcium levels in the energy-dispersive x-ray map. The EDX spectrumresults confirmed the presence of calcium and phosphorous in theelectrospun mats.

FIG. 3A shows a scanning electron micrograph of one embodiment of anelectrospun mat containing 30% HA-TCP using methylene chloride assolvent. FIG. 3B shows corresponding energy dispersive x-ray mapping ofcalcium levels of the mat shown in FIG. 3A as an indicator of thelocation of hydroxyapatite. The SEM shows that there were smallerdiameter fibers forming a web-like network in between larger diameterfibers. The larger diameter fibers were micron scale with averagediameter increasing from about 20 microns to 50 microns with theincrease in ceramic concentration, while the smaller diameter fiberswere in nanometer range. The inter-fiber space between large size fibersis around 100 microns. Calcium mapping results obtained by SEM-EDXshowed that the ceramic was uniformly distributed in the mat (themapping was stopped after 30 minutes as the data were sufficient forthis purpose).

FIG. 4A shows a scanning electron micrograph of one embodiment of anelectrospun mat containing 30% HA-TCP formed using methylene chlorideand dimethylformamide as solvent. FIG. 4B shows corresponding energydispersive x-ray mapping of calcium levels of the mat shown in FIG. 4Aas an indicator of the location of hydroxyapatite. Calcium mappingresults obtained by SEM-EDX for these 30% (HA/β-TCP)-PCL electrospunmats showed that the ceramic was uniformly distributed in the mat (themapping was stopped after 90 minutes as the data was sufficient toassess that ceramic was uniformly distributed).

The fiber size and inter fiber size distance details for electrospunmats fabricated using methylene chloride (MC) as solvent and methylenechloride (MC) and dimethyl formamide (DMF) as solvent are shown in Table1 and Table 2 respectively.

TABLE 1 Fiber size and inter fiber size details of electrospun matsfabricated using MC solvent (Procedure I) 10% HA-TCP 20% HA-TCP 30%HA-TCP 40% HA-TCP 50% HA-TCP Fiber 21.87 ± 5.97   23.72 ± 6.92 28.49 ±8.94  45.69 ± 17.26  47.14 ± 14.30 Diameter (μm) Inter fiber 93.56 ±30.53 117.06 ± 43.6 149.76 ± 40.28 156.12 ± 67.37 141.97 ± 51.73distance (μm)

The fiber diameter and inter fiber distance details of fibers formedusing methylene chloride: dimethylformamide in the ratio of 80:20 areshown in Table 2. The fibers were mostly uniform up to about 30% ceramicloading, but significant agglomerates of the ceramic started wereobserved to form from about 40% ceramic loading. The diameter of thefibers was significantly decreased in Procedure II compared to procedureProcedure I. There was a slight increase in the diameter of the fiber asthe ceramic loading increased. Fibers were of uniform size in procedureProcedure II mats.

TABLE 2 Fiber size and inter fiber size details of electrospun matsfabricated using MC and DMF solvent (Procedure II) 0% 10% 20% 30% HA-TCPHA-TCP HA-TCP HA-TCP Fiber Diameter 2.00 ± .37 2.14 ± .35   2.23 ± .422.46 ± .75 (μm) Inter fiber 14.28 ± 5.62 16.8 ± 2.87 18.90 ± .02 21.09 ±5.46 space ((μm))

Thermogravimetric Analysis

Thermogravimetric analysis (TGA, TA Q50) was used to obtain quantitativeinformation as to the amount of ceramic present in the scaffold as wellas confirmation that there was no solvent in the electrospun mat. Thethermogravimetric analyzer measures the amount and rate of weight changein a material as a function of temperature in a controlled atmosphere. Aheating rate of 10° C. per minute and maximum temperature of 500° C. wasused for this study. Before 500° C., the PCL in the sample is expectedto completely degrade. The remaining weight will be the ceramic.

The thermogravimetric analysis (TGA) profile of the 20/80 HA/TCP-PCLelectrospun scaffolds fabricated using Procedure II is shown in FIG. 5.The data shown on the right axis is the weight percentage of materialremaining at high temperature. The results showed that there is nosubstantial level of solvent left in the electrospun scaffolds. The PCLlikely is degraded fully before 500° C., and this data confirms that theweight percentage remaining at that temperature is approximately theamount of ceramic added to the starting solution. As the sample wasselected randomly for testing, the data also suggests that the ceramicis uniformly dispersed throughout the mat. The profile of the weightloss also shows that the pure PCL mat degrades at a higher temperaturethan scaffolds containing ceramic and as the ceramic concentrationincreases, the material degrades at a progressively lower temperatures.This suggests that the thermal degradation of PCL electrospun mats isbeing catalyzed by ceramic.

In order to better understand this behavior, the thermal degradationprofile of only HA electrospun composite (10%), only β-TCP electrospuncomposite (10%) and 20HA/800-TCP electrospun composite (10%) werecompared to degradation of PCL-only electrospun mats. FIG. 6 shows athermogravimetric degradation profile of electrospun samples made usingdimethylformamide and methylene chloride as solvent (Procedure II). FIG.6 shows that β-TCP accelerates the thermal degradation of the HA/TCP matand not HA. Similar behavior also was observed in mats formed underProcedure I. Without being limited by theory, β-TCP also may acceleratethe degradation of PCL electrospun composite under hydrolyticconditions.

Differential Scanning Calorimeter (DSC)

(1) Crystallization Behavior

FIG. 7 shows the crystallization behavior of electrospun compositesformed using methylene chloride (MC) as solvent and FIG. 8 shows thecrystallization behavior of electrospun composites formed usingmethylene chloride (MC) and dimethylformamide (DMF) as solvent(Procedure II). The heat of fusion (H_(f)) for crystallization andcrystallization temperature values are labeled on the graphs. As shownin FIG. 7, the crystallization temperature (Tc) increased with theaddition of the ceramic. The increase of Tc indicates that thenanoceramics acted as a nucleating agent. This effect was more prominentfor the Procedure I mats (FIG. 7) than for the Procedure II mats (FIG.8). The nucleating effect appears to be reduced slightly with theincrease in the ceramic concentration incase of Procedure I mats,whereas the nucleating effect was increased slightly in the case ofProcedure II mats until 30% concentration was reached. Because it hasbeen shown that uniform dispersion of the ceramic in a composite isnecessary for the ceramic to act as a nucleating agent [Jiann-Wen Huang.Isothermal crystallization of high-density polyethylene and nanoscalecalcium carbonate composites J of applied polymer science. 2007; 107:3163-3172], these observations suggest that the uniformity of theceramic is reduced slightly in the case of mats fabricated usingProcedure I, whereas the uniformity of the ceramic is not affected incase of mats fabricated using Procedure II until 30% ceramicconcentration is reached.

A decrease in the heat of fusion required was observed forcrystallization as the concentration of ceramic increased (values shownon the graph, FIG. 8), which may be related to the concentration of PCLpresent in the composites. As indicated in Table 3, the rate ofcrystallization decreased with the addition of ceramic. Without beinglimited by theory, the reduction in the crystallization rate may takeplace because the interaction between the particles and polymer mayretard molecular mobility. This, in turn, may retard the crystallizationrate [Tamaki Miyazaki, Sumie Yoshioka, Yukio Aso, Toru Kawanishi.Crystallization rate of amorphous nifedipine analogues unrelated to theglass transition temperature. Int'l J. Pharmaceutics. 2007; 336:191-195;Li Sun, Jin-Tao Yang, Gen-Yao Lin, Ming-Qiang Zhong. Crystallization andthermal properties of polyamide 6 composites filled with differentnanofillers. Materials Letters. 2007; 61: 3963-3966; Sha-Ni Lia,Zhong-Ming Lia, Ming-Bo Yanga, Zong-Qian Hua, Xiang-Bin Xua, Rui Huanga.Carbon nanotubes induced nonisothermal crystallization of ethylene-vinylacetate copolymer. Materials Letters. 2004; 58:3967-3970].

TABLE 3 Data of glass transition, crystallization rate and crystallinityof electrospun composites Tg Crystallization Tg (MC + Crystallizationtime (tc) (MC) DMF) time (tc) (MC) (MC + DMF) Crystallinity (%)Crystallinity (%) ° C. ° C. (min⁻¹) (min⁻¹) (MC) (MC + DMF) Only PCL−53.7 −54.0 0.80 0.79 43.5% 40.6% 10% HT −52.0 −52.8 0.46 0.56 43.5%42.0% 15% HT — −51.0 — 0.52 — — 20% HT −49.8 −47.5 0.43 0.50 44.3% 42.0%25% HT — −44.0 — 0.44 — — 30% HT −43.4 −47.6 0.41 0.46 45.0% 43.4% 40%HT −44.2 −52.8 .46 0.47 45.5% 43.5% 50% HT −54.4 −53.8 .54 0.49 44.8%42.6%

(2) Melting Behavior

FIG. 9 shows the melting behavior of electrospun composites formed usingmethylene chloride as solvent. (Procedure I), and FIG. 10 shows themelting behavior of electrospun composites formed using methylenechloride and dimethylformamide as solvent (Procedure II). The heat offusion (H_(f)) for crystallization and crystallization temperaturevalues are labeled on the graphs.

Based on these data, the melting point of PCL electrospun mat has beenestimated to be 56° C. There was a slight increase in the melting pointof composites prepared according to Procedure I and Procedure IIcompared to PCL alone, which suggests that the crystal size of PCL isnot affected by the ceramic. However, there is a significant decrease inthe heat of fusion required for melting as the concentration of ceramicincreased, which may be related to the weight percentage of polymer inthe composite.

The crystallinity of the electrospun scaffolds was calculated from theirheat of fusion for melting values. The 100% crystallinity value of PCLused for calculating the crystallinity of the mats is 138 KJ/g. There isa slight increase in the crystallinity of the samples with the increasein the ceramic concentration when corrected for the weight of theceramic. Also, the crystallinity of mats fabricated using MC (ProcedureI) is slightly more crystalline than mats fabricated using MC and DMF(Procedure II) (Table 3).

Thermally Stimulated Current

As the glass transition (Tg) of PCL was not visible using DSC technique,a highly sensitive technique thermally stimulated current (TSC)technique was used to determine the glass transition. The Tg value ofonly PCL electrospun mat was found to be −54° C. [Table 3]. In the caseof composites fabricated using Procedure I, the glass transition valuesof the composites increased with the increase in ceramic concentrationup to 30% ceramic level, and started to decrease with further increasein the ceramic level, where as in case of mats fabricated usingProcedure II, the Tg values increased till up to 25% ceramicconcentration level and started to decrease with further increase inceramic level. There is a maximum increase of about 10° C. in the glasstransition values of electrospun composite compared to the pure PCL mat.

Porosity and Pore Size

The porosity of the samples was determined from their measured densityvalues and their pore size was determined using the mercury porosimetry.The density of the electrospun mat (Dmat) was calculated by weighing tensamples from each mat. The density values given by manufactures for PCLpellet is 1.14 gm/cm³ and HA and β-TCP are 3.16 gm/cm³ and 3.06 gm/cm³.The density of the raw material based on the composition used wascalculated using the formula: Draw=1/((conc. of ceramic/ρceramic)+(conc.of PCL/ρPCL). The porosity was calculated using the formula: porosity(%)=(1−Dmat/Draw)*100 [Guobao Wei, Peter Ma. Structural and propertiesof nano-hydroxyapatite/polymer composite scaffolds for bone tissueengineering. Biomaterials. 2004; 25: 4749-4757].

The porosity of the MC and MC+DMF electrospun composites is around 78%,and there is no correlation observed in the loading of the ceramic andporosity. However, the mats using MC and the mats made using MC+DMF werehighly differing in the pore size, which was determined using mercuryporosimetry. The mean pore size of a pure PCL mat is about 2.5 microns,30% HA-TCP composite made using MC is about 80.0 microns, and 30% HA-TCPcomposite made using MC+DMF is 7.0 microns.

Example 2 Incorporation of Stem Cells into a Ceramic-Polymer Matrix ofthe Present Invention

In some embodiments, osteoblasts or mesenchymal stem cells areincorporated into the ceramic-polymer scaffold. In some suchembodiments, the electrospun ceramic-polymer matrix is a scaffold fortissue engineering in vivo. In some such embodiments, the electrospunceramic-polymer matrix provides a culturing medium in vitro.

Cell Proliferation

Human MSCs are isolated from adult, human whole bone marrow according tostandard techniques and are seeded onto the ceramic-polymer scaffolds ofthe present invention grown in standard growth medium (DMEM, 10% fetalbovine serum, 1% antibiotic/antimycotic) for 14 days. Cell proliferationis assessed using Vybrant's MTT Cell Proliferation Assay Kit (MolecularProbes, Inc.).

Osteogenic Differentiation.

Bioactive ceramic-polymer scaffolds are created by the process ofelectrospinning, and human mesenchymal stem cells are grown on thescaffolds to determine whether the scaffolds support osteogenicdifferentiation.

hMSCs are grown in control medium (DMEM, 10% FBS, 1% antibiotic) orosteogenic inducing medium (OS) (Control medium with 100 nMdexamethasone, 10 mM b-glycerophosphate,0.05 mM L-ascorbicacid-2-phosphate) on bioactive ceramic-polymer scaffolds created byelectrospinning.

On the day of cell seeding, scaffolds are soaked first in 100% ethanolfor 20 minutes, and then three times in PBS, 20 minutes each, forsterilization. Scaffolds then are placed into assigned wells of a96-well microtiter plate (B-D Falcon, Becton-Dickinson, Inc.) for eachtime point using forceps, and 150 μL of medium containing 10,000 cellsare added to each well. The cells are left in the incubator overnight at37 degrees C. to allow cell attachment to the scaffolds. Media arechanged the next day so that half of the wells receive control mediumand the other half receive osteogenic induction medium (OS). The mediaare changed twice a week thereafter.

While the present invention has been described with reference to thespecific embodiments thereof, it should be understood by those skilledin the art that various changes may be made and equivalents may besubstituted without departing from the true spirit and scope of theinvention. In addition, many modifications may be made to adapt aparticular situation, material, composition of matter, process, processstep or steps, to the objective, spirit and scope of the presentinvention. All such modifications are intended to be within the scope ofthe claims appended hereto.

1. An implantable scaffold for use in tissue engineering or as animplantable material comprising a three-dimensional matrix of micronsized electrospun fibers, wherein the electrospun fibers are formed froman electrospun composite comprising a bioactive ceramic component and apolymer component.
 2. The implantable scaffold according to claim 1,wherein the electrospun composite contains at least 10% by weight of theceramic component.
 3. The implantable scaffold according to claim 1,wherein the electrospun composite contains at least about 60% by weightof the polymer component.
 4. The implantable scaffold according to claim1, wherein the electrospun composite contains at least 10% by weight ofthe ceramic component and at least about 60% by weight of the polymercomponent.
 5. The implantable scaffold according to claim 1, wherein thebioactive ceramic component contains two calcium phosphate ceramicsubstances for every polymer of the polymer component of the composite.6. The implantable scaffold according to claim 5, wherein the calciumphosphate ceramic substances of the bioactive ceramic component of theelectrospun composite are selected from the group consisting oftetracalcium phosphate, amorphous calcium phosphate, alpha-tricalciumphosphate, beta-tricalcium phosphate, and hydroxyapatite.
 7. Theimplantable scaffold according to claim 5, wherein the two calciumphosphate ceramic substances of the bioactive ceramic component of theelectrospun composite are hydroxyapatite and tricalcium phosphate. 8.The implantable scaffold according to claim 7, wherein the bioactiveceramic component of the electrospun composite comprises 20%hydroxyapatite and 80% tricalcium phosphate.
 9. The implantable scaffoldaccording to claim 1, wherein the polymer component of the electrospuncomposite is at least one polymer selected from the group consisting ofa nondegradable polymer and a degradable polymer.
 10. The implantablescaffold according to claim 9, wherein the nondegradable polymer isselected from the group consisting of a polyurethane, a polyvinylidinefluoride, and a polyvinylidine fluoride trifluoroethylene.
 11. Theimplantable scaffold according to claim 9, wherein the degradablepolymer is selected from the group consisting of a poly(lacticacid-glycolic acid), a poly(lactic acid), a poly(glycolic acid), apoly(orthoester), a poly(phosphazene), poly(or polycaprolactone, apolyamide, a polysaccharide, and a collagen.
 12. The implantablescaffold according to claim 1, wherein the polymer of the polymercomponent of the electrospun composite is polycaprolactone.
 13. Theimplantable scaffold according to claim 1, wherein the three dimensionalmatrix of electrospun fibers comprises micron-sized pores.
 14. Acomposition for preparing an implantable osteoinductive andosteoconductive three-dimensional matrix of micron sized electrospunfibers, the composition comprising an electrospun composite containing abioactive ceramic component and a polymer component.
 15. The compositionaccording to claim 14, wherein the electrospun composite of thecomposition contains at least 10% by weight of the ceramic component.16. The composition according to claim 14, wherein the electrospuncomposite of the composition contains at least about 60% by weight ofthe polymer component.
 17. The composition according to claim 14,wherein the electrospun composite of the composition contains at least10% by weight of the ceramic component and at least about 60% by weightof the polymer component of the composite.
 18. The composition accordingto claim 14, wherein the bioactive ceramic component of the electrospuncomposite of the composition contains two calcium phosphate ceramicsubstances for every polymer in the polymer component.
 19. Thecomposition according to claim 18, wherein the calcium phosphate ceramicsubstances of the bioactive ceramic component of the electrospuncomposite of the composition are selected from the group consisting oftetracalcium phosphate, amorphous calcium phosphate, alpha-tricalciumphosphate, beta-tricalcium phosphate, and hydroxyapatite.
 20. Thecomposition according to claim 18, wherein the two calcium phosphateceramic substances of the bioactive ceramic component of the electrospuncomposite of the composition are hydroxyapatite and tricalciumphosphate.
 21. The composition according to claim 20, wherein thebioactive ceramic component of the electrospun composite of thecomposition comprises 20% hydroxyapatite and 80% tricalcium phosphate.22. The composition according to claim 14, wherein the polymer componentof the electrospun composite of the composition is at least one polymerselected from the group consisting of a nondegradable polymer and adegradable polymer.
 23. The composition according to claim 22, whereinthe nondegradable polymer is selected from the group consisting of apolyurethane, a polyvinylidine fluorides and a polyvinylidine fluoridetrifluoroethylene.
 24. The composition according to claim 22, whereinthe degradable polymer is selected from the group consisting ofpoly(lactic acid-glycolic acid), poly(lactic acid), poly(glycolic acid),a poly(orthoester), a poly(phosphazene), a polycaprolactone, apolyamide, a polysaccharide, and a collagen.
 25. The compositionaccording to claim 14, wherein the polymer of the polymer component ofthe electrospun composite of the composition is polycaprolactone.
 26. Amethod of preparing an osteoinductive and osteoconductive scaffold tofacilitate bone repair, the method comprising the steps: (a) preparing aceramic-polymer composite comprising a bioactive ceramic component and apolymer component; (b) electrospinning the ceramic-polymer composite,and (c) thereby depositing a three-dimensional nonwoven matrix ofelectrospun fibers comprising the ceramic-polymer composite on acollector.
 27. The method according to claim 26, wherein the electrospuncomposite of step (a) contains at least 10% by weight of the ceramiccomponent.
 28. The method according to claim 26, wherein the electrospuncomposite of step (a) contains at least about 60% by weight of a polymercomponent.
 29. The method according to claim 26, wherein the electrospuncomposite of step (a) contains at least 10% by weight of the ceramiccomponent and at least about 60% by weight of the polymer component. 30.The method according to claim 26, wherein the bioactive ceramiccomponent of the composite of step (a) contains two calcium phosphateceramic substances for every polymer in the polymer component of thecomposite.
 31. The method according to claim 30, wherein the calciumphosphate ceramic substances of the bioactive ceramic component of thecomposite of step (a) are selected from the group consisting oftetracalcium phosphate, amorphous calcium phosphate, alpha-tricalciumphosphate, beta-tricalcium phosphate, and hydroxyapatite.
 32. The methodaccording to claim 30, wherein the two calcium phosphate ceramicsubstances of the bioactive ceramic component of the composite of step(a) are hydroxyapatite and tricalcium phosphate.
 33. The methodaccording to claim 32, wherein the bioactive ceramic component of thecomposite of step (a) comprises 20% hydroxyapatite and 80% tricalciumphosphate.
 34. The method according to claim 26, wherein the polymercomponent of the composite of step (a) is at least one polymer selectedfrom the group consisting of a nondegradable polymer and a degradablepolymer.
 35. The method according to claim 34, wherein the nondegradablepolymer is selected from the group consisting of a polyurethane, apolyvinylidine fluoride, and a polyvinylidine fluoridetrifluoroethylene.
 36. The method according to claim 34, wherein thedegradable polymer is selected from the group consisting of poly(lacticacid-glycolic acid), poly(lactic acid), poly(glycolic acid), apoly(orthoester), a poly(phosphazene), a polycaprolactone, a polyamide,a polysaccharide, and a collagen.
 37. The method according to claim 26,wherein the polymer of the polymer component of the composite of step(a) is polycaprolactone.
 38. The method according to claim 26, whereinthe electrospun fibers of step (c) are micron sized.
 39. The methodaccording to claim 26, wherein the matrix of electrospun fibers in step(c) comprises micron-sized pores.
 40. The method according to claim 26,further comprising the steps: (d) seeding the three-dimensional nonwovenmatrix of electrospun fibers with isolated differentiable humanmesenchymal cells or osteoblasts; and (e) growing the differentiablehuman mesenchymal cells or osteoblasts on the three-dimensional nonwovenmatrix of electrospun fibers so that the differentiable humanmesenchymal cells or osteoblasts differentiate into a mature cellphenotype on the scaffold.